<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        //1,新增数组元素  修改length长度
        var arr = [1, 2, 3];
        console.log(arr.length);
        arr.length = 5;
        console.log(arr);
        console.log(arr[3]); //undefined
        //2，新增数组元素  修改索引号
        var arr1 = [1, 2, 3];
        arr1[3] = 4;
        console.log(arr1);
    </script>
</head>

<body>

</body>

</html>